NO324695B1 - Process for the preparation of pure saline based on retardants. - Google Patents

Description

The present invention relates to a method for producing very pure salt water based on retarders.

Much of today's salt (mainly NaCl) is produced using evaporation processes where salt is crystallized from salt water. The use of very pure salt water has several advantages in such a process.

Said salt water is typically obtained by dissolving mine rock salt deposits. Rock salt, which mainly originates from maritime deposits, contains alkaline earth metals (such as Ca, Mg or Sr) and potassium salts as the most important contaminants. Sulfate, chloride and bromide are typical counterions. Together with the sulphate ion, calcium will be present as a rather insoluble CaS04 (anhydride) and/or as polyhalite (K2Mg2Ca2(SC>4)4 . 4H2O).

The total amount of calcium and sulphate in rock salt deposits depends on the deposit itself, but can, for example, also vary with the depth where the salt is excavated. Calcium is typically present in an amount of 0.5 to 6 grams per kilogram and sulphate from 0.5 to 16 grams per kilograms. Solution extraction is a technique by which highly soluble salts can be extracted in special places in a deposit. The advantage of this method is that poorly soluble contaminants, such as anhydrite (CaSCu) and gypsum (CaSC»4. 2H2O) will partially remain in the cavity that is extracted. However, the resulting salt water may be saturated with these undesirable contaminants. Without any treatment, the (soil) alkali contamination will cause salt water obtained from one of the aforementioned sources to cause severe crusting in the heating tubes in a vacuum crysallizer for NaCl. Calcium sulphate, which is difficult to remove in several forms, will block the pipes and inhibit heat transfer. Among other things, contamination of the resulting salt and poor energy utilization in the process will be the consequence. Very pure salt water is also of interest for processes in which salt solutions are used as raw materials, such as in the chemical transformation industry, for example the chlorine and chlorate industry. In particular, the conversion from mercury and diaphragm technology to the more environmentally acceptable membrane technology triggered the need for very clean salt water. The salt water for use in these processes is typically obtained by dissolving a salt source which can be rock salt, salt from evaporation processes as described above and/or solar salt, including lake or sea salt. It is noted that sea salt typically contains less than 0.5 g/l CaSC>2 due to the fact that CaSO2 is typically present in the form of gypsum with only a limited solubility.

The use of very pure salt water was found to be of interest to this industry, because it enabled better energy utilization as well as the generation of less waste. Moreover, products originating from the chemical transformation industry can be of high quality if high purity salt water is used to prepare them.

Consequently, many attempts have been made to improve the quality of salt water. The first solution was to use very pure salt that was dissolved to make such salt water. Very pure salt can be achieved by preventing calcium sulfate from crystallizing in the salt production process by adding specific germs or by using a scale inhibitor. US 3,155,458 describes, for example, adding starch phosphate to the brine in the evaporation-crystallization process. It is claimed that starch phosphate increases the solubility of CaSC<4, and thus prevents the deposition and enables the production of salt with a very high purity and low CaS04 content.

However, such a process requires the undesirable bleeding of a CaSO 4 -rich stream from the crystallization process and also requires the brine to be essentially bicarbonate-free.

Another solution is to remove contaminants from the raw salt water by chemical treatment of this salt water. An example of such treatment is given in the already more than 100-year-old Kaiserliches Patentamt DE-115677, in which hydrated lime is used to precipitate magnesium hydroxide and gypsum from the raw salt water.

In addition to, or instead of, these methods, efforts have also been made to increase the purity of the salt water by reducing the amount of contaminants such as the above-mentioned anhydrite, gypsum and polyhalite (and/or their strontium analogues) that dissolve in said salt water. This is typically done by adding certain agents to the water used in the process, or by mixing such agents with the salt source before adding water (especially for solar salt dissolvers). Hereafter, such agents are called "retarding agents". DD-115341 discloses that brine, particularly for use in processes for preparing pot ash, with a reduced amount of CaSC4 and MgSO4 can be obtained by adding calcium lignin sulfonate to the water used to prepare the brine. The addition of calcium lignin sulfonate is claimed to lower the solubility of CaSCu and MgSC«4.

US 2,906,599 describes the use of a group of phosphates, called "polyphosphates" including hexametaphosphates, to reduce the degree of dissolution of calcium sulfate (anhydrite), leading to salt water with reduced sulfate and calcium ions. At lower concentration, (ie up to 50 ppm in the salt water) the hexametaphosphates were found to be the most effective agent, sodium hexametaphosphate being the preferred retarder.

Currently, another type of retarder is marketed by Jamestown Chemical Company Inc. under the name (Sulfate Solubility Inhibitor) SSI®200. According to the data sheet, the material contains dodecylbenzenesulfonic acid, sulfuric acid and phosphoric acid.

The present invention relates to a method for producing very pure salt water based on retarders.

Surprisingly, we found that the use of a specific combination of compounds leads to a reduction of the level of contaminants, especially calcium sulfate, in salt water obtained by dissolving a salt source compared to salt water obtained when common retarders are used. It was observed that the above particularly applies when the salt source is contaminated with anhydrite, gypsum, polyhalite, strontium-containing equivalents of these compounds and/or clay minerals.

Accordingly, the invention relates to a method for producing very pure salt water, in which a salt source contaminated with alkaline earth metal sulfate is dissolved in water in the presence of an effective amount of two or more retarding agents to reduce the level of said alkaline earth metal sulfate dissolved in said salt source, in particular the amount of calcium sulfate is reduced, where said level is reduced so that a maximum of 80% of the alkaline earth metal sulfate is dissolved compared to a blank dissolution step in which the combination of retarders is not used, characterized in that at least one retarder of low molecular weight having a molecular weight of less than 800 Daltons and one retarder of high molecular weight having a molecular weight of at least 1,000 Dalton is used, where the retarder with the lowest molecular weight is selected from the group consisting of:

- alkylbenzene sulphonates, in which the alkyl groups can be linear or branched,

- water-soluble phosphates,

- ethoxylated compounds with one or more sulfite, sulfonate, sulfate, phosphite, phosphonate, phosphate and/or carboxyl groups and - C2-C40 alkyl compounds with one or more sulfite, sulfonate, sulfate, phosphite, phosphonate, phosphate and/or carboxyl groups,

wherein the retarder with a higher molecular weight is selected from the group consisting of compounds with optionally substituted, linear or branched alkyl backbones that are functionalized with sulfonate, sulfate, phosphite, phosphonate, phosphate and/or carboxyl groups,

and where the combination of two or more retarding agents acts synergistically in reducing dissolved alkaline earth metal sulfate.

It was found that the thus obtained very pure salt water could be used without further purification in both the evaporative salt crystallization and the chemical transformation industry. However, if desired, the salt water can be further purified using normal chemical treatment. Also, the use of scale inhibitors and/or specific seeds in the evaporative crystallization technique to prevent CaSO 4 precipitation is no longer required. If desired, however, the scale inhibitors and/or the specific germs can be used in combination with the very pure salt water in the present process, i.e. possibly further purified by chemical treatment.

Furthermore, it was found that the use of retarders according to the invention led to systems which showed very little foaming, a disadvantage which is often observed in ordinary systems comprising sulphonates and the like.

It is noted that the term "salt" used in this document is intended to denote

all salts in which more than 25% by weight is NaCl. Preferably, such salts contain more than 50% by weight of NaCl. Even more preferably, the salt contains more than 75% by weight NaCl, while a salt containing more than 90% by weight NaCl is most preferred. The salt can be solar salt (salt obtained by evaporating water from salt water using solar heat), rock salt and/or underground salt deposits. Preferably, it is an underground salt deposit that is exploited by means of solution extraction. As the different sources of salt provide salt with different compositions, especially with regard to contaminants, the effectiveness of the retarders must typically be judged to optimize their effectiveness.

The effectiveness of the combination of compounds as retarders and whether they are synergistic or not is quickly and easily determined using the following dissolution test method. The salt source is crushed to obtain particles of 0.1 to 1.5 cm. A fresh stock solution of approx. 1000 mg/l retarder compound(s) is prepared, and the desired amount of this stock solution (the amount to be evaluated) is added to a 11 beaker filled with such an amount of mineral-free water that the total volume after adding the stock solution is 660 ml. A blank experiment in which no retarding compound is used is carried out in parallel. The beaker is stirred with a magnetic, Teflon-coated stirring rod with a tapered, round construction and a size of 50x9 mm (supplied from Aldrich Cat. No. Z28.392-4) at 200 rpm and thermostated at 20°C. 300 g of the crushed core sample is added to this solution, and the mixture is continuously stirred at 200 rpm. After 1 hour, samples are taken of the salt water. For this purpose, the magnetic stirrer is stopped and a desired amount of salt water sample is taken and filtered through a 0.2 micron filter. The filtered salt water sample is then analyzed with regard to the amount of dissolved Ca, Mg, K, Sr and/or SC ions. To test the long-term effect of the retardants, the test may continue for several days, preferably more than 5 days. To prevent erosion of the salt source, the mixture is not stirred during this period, and samples are taken once a day. Before sampling, the mixture is stirred by hand for 1 minute using a 4 mm thick glass rod, so that the aqueous phase is homogeneous. The effect of the retarder is defined as the percentage by which the concentration of the relevant ions is reduced and is compared with the blank sample.

The effect is such that a retardation of the dissolution (in m.equivalents/1) of one or more of the alkali metal ions, the alkaline earth metal ions and/or sulfation is more than 20%, preferably more than 30, more preferably more than 40%, even more preferably more than 50%, and most preferably more than 70% is observed compared to the blank. If the long-term effect (after 5 days) is insufficient, preferably the retarder compound with the higher molecular weight is increased in concentration and/or its molecular weight must be increased. If the retardation after 1 day is less than desired, preferably the amount of the low molecular weight compound must be increased.

The amount of retarding agents to be used depends on the quality of the salt source, the quality of the water used to produce the salt water and the type of agents used. In general, the amount of each retarding agent will be less than 0.1%, preferably less than 0.05%, more preferably less than 0.02% by weight of the water, while a concentration of less than 0.01% of each of the compounds is most preferred. Depending on the circumstances, more or less of each retarding compound must be used. Preferably, the weight ratio of the low molecular weight retarder to the high molecular weight retarder is from 100:1 to 1:100. More preferably, the weight ratio between the low molecular weight and high molecular weight retardant is from 20:1 to 1:20.

The outstanding and beneficial synergistic properties of the combination of the compounds are believed to be due to: i) a rapid coverage of the surface of the alkaline earth metal source by the compound

with the lowest molecular weight,

ii) a rapid coverage of the surface of the calcium source in step i) with the retarder of the highest molecular weight (compared to the rate of coverage of an uncovered alkaline earth metal source);

iii) a relatively rapid desorption of the retarder with the lowest

molecular weight, and

iv) hardly any desorption of the retarder with the highest molecular weight;

and

v) interaction of the low molecular weight retarder with the de(t)

apolar segment(s) in the retarder with the highest molecular weight.

Although the combination of retarders is not optimized, we prefer to use a low molecular weight compound with a molecular weight of less than 1,000, more preferably less than 800, even more preferably less than 600, even more preferably less than 500, and most preferably less than 400 Dalton, in combination with a retarder having a higher molecular weight than the low molecular weight compound, preferably having a molecular weight of at least 500, more preferably of at least 600, even more preferably of more than 800, even more preferably more than 1000, and most preferably greater than 1250 Daltons.

Depending on the structure of the high molecular weight retarder, it can have a very high molecular weight up to several million Daltons, provided that the product is still water soluble or water dispersible, for example through the formation of a bi-layer where the polar groups interact with the water. The high and low molecular weight compounds may be any compounds which satisfy the desired retarding action when tested as described above. Compounds with a suitable high molecular weight include lignosulphates, phospholipids, hydrolysed phospholipids, polyacrylates as well as various other compounds with optionally substituted, linear or branched alkyl backbones which are functionalized with sulphonate, sulphate, phosphite, phosphonate, phosphate and/or carboxyl groups, which include less preferred compounds commonly known as flocculants. It is preferred that such compounds have a maximum of 3, more preferably 2, apolar alkyl chains per polar function to achieve the best deceleration properties.

Preferably, the molecular weight of the low molecular weight retarder is the same (+/-20%) as the molecular weight of one of the apolar segments of the high molecular weight material. Although the retarders may comprise two or more ethoxy and/or propoxy repeating units, it is preferred that less than 20, preferably less than 10, even more preferably less than 5, and most preferably less than 3 ethoxy and/or propoxy units are present per molecule in the retarder.

Although different sources and combinations of the retarding agents can be used, it is preferred to use retarding agents that are not environmentally objectionable. More specifically, it is preferred to use retarders present in biomembranes (membranes of naturally occurring organisms). Typically, such derivable retarders are very good retarders with a high molecular weight and are therefore preferably used as such in combination with a low molecular weight compound. Due to their effectiveness as a (high molecular weight) retarder, the use of biomembrane derivable phospholipids and/or hydrolyzed phospholipids is preferred.

It is possible to use a phospholipid mixture containing both low and high molecular weight phospholipids to act as a combination retarding compound. In particular, if such a mixture can be derived from a natural source in a single step, such mixtures may be preferred.

In a most preferred embodiment, the high molecular weight retarder is a high molecular weight phospholipid obtained from yeast cells, especially baker's yeast cells. Such yeast cells can be cultivated on site. If low levels of sugar or other substrate in the salt water are acceptable, one could add a small amount of yeast and sugar (or any other suitable substrate for the yeast) to the water in which the salt is dissolved, so that the yeast is grown just before the dissolution of the salt.

In a most preferred embodiment, the low molecular weight compound is selected from the group consisting of: alkylbenzene sulfonates, in which the alkyl groups may be linear or branched, phosphates, preferably polyphosphates, including alkali metal and ammonium phosphates which are water soluble,

ethoxylated compounds with one or more sulfite, sulfonate, sulfate, phosphite,

phosphonate, phosphate, and/or carboxyl groups, and/or

C2-C40 alkyl groups, preferably C2-C20 alkyl groups, with one or more sulfite, sulfonate, sulfate, phosphite, phosphonate, phosphate and/or carboxyl groups, such as polyacrylic acids.

The term polyphosphate includes metaphosphates, such as hexametaphosphate (Na3P03)6, tripolyphosphates (NasPaOio), tetraphosphates (Na6P40i3), pyrophosphates, such as Na4P207, and

Na2H2P2C>7, as well as various other complex phosphates which are typically derived from ortho-phosphoric acid compounds by molecular dehydration, and mixtures of two or more of these phosphates.

It is noted that the effectiveness of the retarders (mixtures) can be affected

negatively by the presence of contaminants such as biomaterial and/or clay minerals in the process water used to dissolve the salt and/or by the presence of such contaminants in the salt deposit. This problem is believed to originate from adsorption of (parts of) the retarder (mixture) on the surface of these biomaterial and/or clay mineral contaminants. Obviously, removing the contaminants from the water can solve this problem by filtration or from the salt with a solid-solid separation before dissolving the salt. However, it is noted that it is also possible to add a component to the retarder mixture which is specifically adsorbed on the biomaterial and/or clay material contaminants. In this way, the adsorption of the retarder on these contaminants can be prevented, thereby increasing the effectiveness of the retarder (mixture).

Experimental

The low molecular weight retarders:

SDBS (Sodium Dodecylbenzenesulfonate) = Marion® ARL ex Condea Chemie

Na pyrophosphate = sodium pyrophosphate ex J.T. Bakes

The high molecular weight retarders:

Na-lignosulfonate = Darvan ® 2 ex Vanderbilt

Yeast DSM = Dried, inactive baker's yeast ex DSM

Baker's yeast = active yeast used in bakeries = high molecular weight retarder.

Examples 1-3 and Compare six examples A-C

A core from the Akzo Nobel Saltmine in Stade, Germany was subjected to the above mentioned dissolution test method while various retarders were evaluated. The amount of dissolved Ca and SO4 (mEq/1) after 1 and 4 days was determined and the average effect (perf.) calculated. The results are listed in the following table.

In a similar way, a core from the Akzo Nobel salt mine in Delfzijl was evaluated with the following result:

It is clear that the combination of retarder with high and low molecular weight is beneficial for reducing the amounts dissolved in Ca and SO4 ions. Furthermore, the samples with only SDBS showed more of the unwanted foaming than the samples in which retarders according to the invention were used.

Claims (4)

1. Process for producing very pure salt water, in which a salt source contaminated with alkaline earth metal sulfate is dissolved in water in the presence of an effective amount of two or more retarding agents to reduce the level of said alkaline earth metal sulfate dissolved in said salt source, in particular the amount of calcium sulfate is reduced, wherein said level is reduced so that a maximum of 80% of the alkaline earth metal sulfate is dissolved compared to a blank dissolution step in which the combination of retarders is not used, characterized in that at least one retarder with a low molecular weight having a molecular weight of less than 800 Dalton and one retarder with a high molecular weight having a molecular weight of at least 1,000 Dalton is used, where the retarder with the lowest molecular weight is selected from the group consisting of: - alkylbenzene sulfonates, in which the alkyl groups can be linear or branched, - water-soluble phosphates, - ethoxylated compounds with one or more sulfite, sulfonate, sulfate - , phosphite, phosphonate, phosphate and/or carboxyl groups and - C2-C40 alkyl compounds with one or more sulfite, sulfonate, sulfate, phosphite, phosphonate, phosphate and/or carboxyl groups, wherein the higher molecular weight retarder is selected from the group consisting of compounds with optionally substituted, linear or branched alkyl backbones that are functionalized with sulfonate, sulfate, phosphite, phosphonate, phosphate and/or carboxyl groups, and where the combination of two or more retardants act synergistically in reducing dissolved alkaline earth metal sulfate. 2. Method according to claim 1, in which the compound with a higher molecular weight is selected from the group consisting of lignosulphates, phospholipids and polyacrylates, preferably phospholipids. 3. Method according to each of the preceding claims, in which the combination of compounds is a mixture of phospholipids with low or high molecular weight, preferably originating from yeast cells, especially baker's yeast. 4. Method according to each of the preceding claims, in which the salt water is further purified by means of a common chemical treatment.